1. Field of the Invention
The present invention relates to a ferrite steel which excels in high-temperature strength and also in toughness in weld heat affected zones.
2. Description of the Related Art
Hitherto, ferrite steels have been widely used as a heat- and acid-resistant material, due to the fact that such steels have the following advantages (1) to (3) over austenitic steels.
(1) Ferrite steel in general has a small thermal expansion coefficient and excels in characteristics under such conditions in which it undergoes repeated heating cycles, i.e., superior in resistance to thermal fatigue and resistance to repeated oxidation. PA0 (2) Ferrite steel is easy to bond to other parts (steel or cast iron). PA0 (3) Ferrite steel is comparatively inexpensive.
It is to be understood, however, that ferrite steel is generally inferior to austenitic steel in high temperature strength and workability of welded portions, so that this type of steel has only limited uses. Namely, ferrite steel cannot suitably be adopted in uses which require particularly high strength at high temperatures and good workability of weld portions.
For instance, in the field of automotive parts, exhaust pipes are required to sustain high temperatures well exceeding 850.degree. C., sometimes 900.degree. C. or higher, in order to meet the demand for higher engine performance, i.e., demands for increases in engine output power and reduced fuel consumption.
Construction and configuration of exhaust pipes also are becoming complicated, posing a risk of embrittlement cracking at welded parts due to inferior workability of the welded part, during working for realizing such complicated configurations. In general, workability is reduced when the strength is increased. Increases in strength alone cannot provide materials suitable for use as exhaust pipe materials.
Referring now to the base metal of such steels, remarkable improvement has been achieved in recent years due to reduction in C and N contents and addition of stabilizing elements such as Nb and Ti. When a metal is subjected to welding, the toughness of the portion which is molten by welding heat can appreciably be improved by suitable selection of the welding rod material. However, the toughness of the heat affected zone of the base metal is substantially influenced by the composition of the base metal. Therefore, it has been extremely difficult to develop a material which simultaneously exhibits large high-temperature strength and high toughness of the heat affected zone.
Hitherto, various materials have been proposed to cope with such a problem. For instance, Japanese Patent Publication No. 1-41694 discloses that creep characteristics can be improved by addition of Nb in excess of a predetermined amount, i.e., by making the material contain an effective amount of Nb. It is understood that the material proposed in this Japanese Patent Publication is improved also in high-temperature strength. This material would be suitably employed for use which does not require a specifically high level of toughness at the weld portion. Thus, the above-mentioned Japanese Patent Publication fails to teach or suggest improvement in toughness.
Japanese Patent Laid-Open No. 57-85960 discloses an Nb-containing ferrite steel. The art disclosed in this Laid-Open specification appreciably improves toughness of the ferrite steel but does not give specific consideration to the improvement of high-temperature strength. In fact, in this Laid-Open specification, the content of Nb, which is an element important for attaining improvement in high-temperature strength, is limited to be not more than 0.45 wt % (last line, page 12 to line 1, page 13) from the view point of toughness, preferably between 0.25 and 0.4 wt %. When the above-mentioned upper limit of Nb content is exceeded, toughness of the steel is drastically reduced as shown in FIG. 2 of this application. In addition, the above-mentioned Laid-Open specification states that the Al content is preferably 0.5 wt % or less from the view point of toughness.
In general, when Nb content exceeds a chemical stoichiometric value for bonding to C and N expressed by (C.times.93/12+N.times.93/14), Nb is preferentially bonded to C and N so that Al exists in a dissolved state. As well known to those skilled in the art, dissolved Al impairs workability and toughness.
Japanese Patent Laid-Open No. 56-25953 discloses a ferrite steel which excels in resistance to high-temperature oxidation and creep, as well as in weldability. The steel shown in this Laid-Open specification essentially contains Al by an amount not less than 0.5 wt % in order to exhibit improved resistance to oxidation. It is stated, however, that the Al content should not exceed 2 wt %, because Al adversely affects weldability (line 14, page 17). Thus, the art disclosed in this Laid-Open specification gives a preference to improvement in the oxidation resistance at high temperature and proposes to add Al by an amount not less than 0.5 wt % even though addition of Al is not desirable.